U.S. Pat. Nos. 3,620,933 and 3,666,549 describe novel ways to produce printed circuts additively, and the methods taught are now used commercially on large scale. Essentially the methods comprise first producing a laminate of specially treated metal foil and a polymerized resin support or substrate. The metal foil of this initial laminate is a sacrificial foil, in that it is completely removed prior to use of the substrate in the electroless plating operation. Apart from the mechanical effect of protecting the surface of the substrate against scratching, dirt, or other contamination prior to removal of the foil, the foil serves primarily as a means of modifying the nature of the substrate surface so that it becomes highly receptive, on chemically dissolving the foil from the surface, to metal deposits applied by known electroless plating techniques, or other deposition procedures such as vacuum metalizing for example. The inventions disclosed in these patents are especially useful in the manufacture of printed circuit boards because of the protective barrier offered by the sacrificial metal foil, prior to use of the laminates in producing the finished circuit boards. But some difficulty has been encountered in obtaining adequate adhesion, particularly in respect to thermal shock resistance, of circuit boards produced from such substrates by what is called an all-additive electroless plating process unless the pre-plating and plating operations are interrupted to introduce a baking of the boards.
There are two ways to produce a printed circuit board using the techniques described in the above patents. One, the so-called "semi-additive" method, consists in catalyzing the whole surface of the pretreated substrate, metalizing the entire surface of the laminate with a thin metal film by electroless deposition, and then applying an organic coating or resist in a defined pattern or mask to outline the desired circuit configuration on the board. The plating operation is interrupted at this point to dry and then bake the board to develop good adhesion of the initial metal film to the substrate. The plating operations are then resumed, and an electrolytic plating operation, using this initial metal film as an electrical bus bar on the substrate surface, is then employed to build up additional metal, usually copper and solder, to the desired total thickness in the unmasked areas which will serve as the circuit board conductors. After this has been done, the organic resist is removed and the unwanted (originally masked and therefore still thin-film) areas of the electrolessly deposited metal are etched away chemically, leaving only the heavier electroplated areas to define the printed circuit.
The other method, which is the above-mentioned "all-additive" method, involves no initial deposition of the "bus bar" film and subsequent etching away of unwanted portions of it. In this method, the entire substrate is also catalyzed and dried, but before any metal is deposited, a standard organic polymer resist coating is applied directly on the substrate surface to define the non-circuit areas. The resulting panel is then treated with a dilute acid or base to reactivate or accelerate unmasked circuit areas, and the panel is immersed in an electroless plating solution where is is retained until the total desired thickness of electroless metal deposit is built up. Baking at some point prior to completion of the plating operations is again needed to develope adequate peel strength or bond between the plastic and metal, especially where the board is subject to thermal shock as occasioned by soldering operations.
It is also possible to employ a different technique of preparing substrates for all-additive plating in accordance with the teaching of U.S. Pat. No. 3,698,940. In this procedure no sacrificial metal cladding is used to modify the substrate surface; instead the resin substrate is first subjected to a particular type of organic solvent etching, followed by treatment in a hexavalent chromic-sulfuric acid solution. These boards may then be electrolessly plated by either the semi-additive or the all-additive procedures described above. While circuit boards of this type produced by the all-additive plating technique are somewhat better in terms of peel strength or adhesion of plated metal to plastic than are boards produced from the sacrificial metal clad laminates, a baking operation prior to completion of the plating operations is again required to develope acceptable bond strength. Moreover there is a practical disadvantage of the organic solvent-acid etched substrates in that the activated surface is unprotected so that care must be exercised to prevent it from becoming scratched, dirty or otherwise contaminated before being electrolessly plated. This means that the plater himself must perform the surface preparation operations which introduce difficulties of fire hazard with the organic solvent and pollution resulting from the chromic acid etch. These difficulties are avoided by use of the sacrificial metal clad laminates since a plastic fabricator can prepare the laminates in bulk, store and then ship it to the plater as needed. All the plater must do is chemically strip the sacrificial metal from the surface, as by a simple inorganic acid bath, to ready the boards for the plating operations.
While the all-additive plating processes taught by the foregoing patents has been commercially attractive, it was found that difficulties arose in meeting the more severe adhesion tests that industry has come to require.